Publication Date:
2008-09-01
Description:
Biomarker and stable carbon isotope ratios were used to infer the age, lithology, organic matter input, and depositional environment of the source rocks for 388 samples of produced crude oil, seep oil, and tarballs to better assess their origins and distributions in coastal California. These samples were used to construct a chemometric (multivariate statistical) decision tree to classify 288 additional samples. The results identify three tribes of 13C-rich oil samples inferred to originate from thermally mature equivalents of the clayey-siliceous, carbonaceous marl and lower calcareous-siliceous members of the Monterey Formation at Naples Beach near Santa Barbara. An attempt to correlate these families to rock extracts from these members in the nearby COST (continental offshore stratigraphic test) (OCS-Cal 78-164) well failed, at least in part because the rocks are thermally immature. Geochemical similarities among the oil tribes and their widespread distribution support the prograding margin model or the banktop-slope-basin model instead of the ridge-and-basin model for the deposition of the Monterey Formation. Tribe 1 contains four oil families having geochemical traits of clay-rich marine shale source rock deposited under suboxic conditions with substantial higher plant input. Tribe 2 contains four oil families with traits intermediate between tribes 1 and 3, except for abundant 28,30-bisnorhopane, indicating suboxic to anoxic marine marl source rock with hemipelagic input. Tribe 3 contains five oil families with traits of distal marine carbonate source rock deposited under anoxic conditions with pelagic but little or no higher plant input. Tribes 1 and 2 occur mainly south of Point Conception in paleogeographic settings where deep burial of the Monterey source rock favored petroleum generation from all three members or their equivalents. In this area, oil from the clayey-siliceous and carbonaceous marl members (tribes 1 and 2) may overwhelm that from the lower calcareous-siliceous member (tribe 3) because the latter is thinner and less oil-prone than the overlying members. Tribe 3 occurs mainly north of Point Conception where shallow burial caused preferential generation from the underlying lower calcareous-siliceous member or another unit with similar characteristics. In a test of the decision tree, 10 tarball samples collected from beaches in Monterey and San Mateo counties in early 2007 were found to originate from natural seeps representing different organofacies of Monterey Formation source rock instead from one anthropogenic pollution event. The seeps apparently became more active because of increased storm activity. 2nd revised manuscript received April 11, 2008 Kenneth Peters has a Ph.D. in geochemistry from University of California, Los Angeles, and has used numerical modeling and geochemistry to study petroleum systems at the U.S. Geological Survey since 2002. He spent 15 years with Chevron and 9 years with Mobil and ExxonMobil and taught petroleum geochemistry and basin modeling at Chevron; Mobil; ExxonMobil; Oil and Gas Consultants International; University of California, Berkeley; and Stanford University. Ken is the principal author of The Biomarker Guide (2005, Cambridge University Press) and is currently chair of the AAPG Research Committee. Frances Hostettler has a masters degree from University of Wisconsin and has worked in the field of environmental organic geochemistry for 31 years. She worked on chemotaxonomy at the U.S. Department of Agriculture Forest Products Laboratory for four years, studied priority pollutants at a municipal water treatment plant, and has worked on pollution problems and oil spills at the U.S. Geological Survey for the past 23 years. Thomas Lorenson has been a geologist and research geochemist with the U.S. Geological Survey in Menlo Park, California since 1989. He has been actively studying oil and gas seeps in California and around the world with particular interest in gas hydrates and oil biomarker studies. He holds an M.S. degree in geology from San Diego State University. Robert Rosenbauer is a senior geologist with 33 years of research in organic, inorganic, and experimental geochemistry of fundamental geologic processes, mostly of submarine hydrothermal systems. His recent studies aim to understand natural and human-induced stresses on the environment, including natural and pollutant hydrocarbons, the carbon cycle, experimental multiphase CO 2-H 2O brine-rock interactions, and ecosystem restoration in coastal settings. He has authored more than 80 articles on experimental investigations, theoretical modeling, and basic research.
Print ISSN:
0149-1423
Electronic ISSN:
1943-2674
Topics:
Geosciences
Permalink